Soft decks can be indicators of several problems, some serious, some not so much. I suggest that you research fiberglass boat repairs and study or buy some of the excellent articles, books, on fiberglass work.
The deck is gelcoated fiberglass (including matting and resin) laid over either marine plywood, a balsa core, or some other core. Older decks are probably balsa cored. The underlaying bottom skin is fiberglass/resin and fiberglass cloth and/or chopped fiberglass mat material. The sandwich is really held together by the resin impregnating process. The last deck which I repaired was on a Cape Dory Typhoon (about 19 feet) which was delaminating and also experiencing core rot due to moisture penetration. It was a 1974 model and had a few through hull and mounted fittings. It took only a little lack of maintenance by the previous owner to allow the moisture to seep throughout the core. The deck was mostly nonskid patterned with smooth surfaces around the edges, maybe two inches wide. Short story is that we used a circular saw and a Dremel tool to cut around the deck at the edge of the non slip pattern, cutting only as deep as the top sandwich layer/skin. We were able to lift off the layer like taking off a skin. The balsa core was rotting and looked more like charcoal. The bottom skin was holding everything together. We totally removed the balsa that was exposed and gouged out as much as possible under the smooth surface. Using the top skin as a pattern, we shaped marine plywood, same thickness as the original balsa, to fit the area we had exposed. The new core was resin coated along with fiberglass cloth and glued to the bottom sandwich layer. Remaining exposed voids were filled with resin/epoxy/cloth. Then, the top skin was glued to the new core using resin epoxy and fiberglass cloth and shreds. The gap between the smooth edge and the skin was sealed/filled with epoxy mixed with some small amount of filler. Sanding and fairing finished off the surface. All surfaces were prepped as much as possible and the deck was painted with Interlux marine paint. Later, through hulls, fittings, etc. were replaced and properly bedded. We used West System epoxy/resin/fiberglass system. This same process was repeated on the stern deck, and some of the side areas. And, this is a short version of what was involved. Boat looked great when finished, was very solid, no seams were showing, and gave no problems as long as I kept it; I sold it a few years ago. Which led to me ordering a 2016 Catalina 14.2 with keel; best small dinghy sailer I have ever owned, rigged basically just like the Cape Dory and it's predecessor Tanzer 22.
Also, look into minor deck delamination information and you may find that injections of resin in holes which you carefully drill will solidify minor delamination areas.
Point to all of this is, consider your abilities/experience with fiberglassing techniques and study, study, study. The above worked because the inner liner/bottom skin was solid and in good shape, and we were able to examine all areas for strength and flex integrity.
Good luck!